During the past several years, the popularity and viability of fuel cells for producing both large and small amounts of electricity has increased significantly. Fuel cells conduct an electrochemical reaction with reactants such as hydrogen and oxygen to produce electricity and heat. A typical fuel cell includes an electrolyte disposed between two electrodes: an anode and a cathode. Fuel cells are usually classified by the type of electrolyte used into one of five groups: proton exchange membrane (PEM) fuel cells, alkaline fuel cells (AFC), phosphoric-acid fuel cells (PAFC), solid oxide fuel cells (SOFC), and molten carbonate fuel cells (MCFC). While all fuel cells have some desirable features, solid oxide fuel cells (SOFC) have a number of distinct advantages over other fuel cell types. Some advantages of SOFCs include reduced problems with electrolyte management, increased efficiencies over other fuel cell types (SOFCs are up to 60% efficient), higher tolerance to fuel impurities, and the possible use of internal reforming or direct utilization of hydrocarbon fuels.
Most SOFCs include an electrolyte made of a solid-state material, such as a ceramic, capable of quickly conducting oxygen ions. In order to promote ionic conductivity in the electrolyte, SOFCs typically operate in the 500° to 1000° C. temperature range. An oxidant, such as air, is fed to the cathode, which then creates and supplies oxygen ions to the electrolyte. A fuel such as hydrogen or methane is fed to the anode where the fuel reacts with oxygen ions transported through the electrolyte from the cathode. This reaction produces electrons which are then delivered to an external circuit as useful power. To increase the amount of usable power, multiple fuel cells are grouped in arrays or fuel cell stacks on ceramic substrates. These stacks are, in turn, layered together to form fuel cell systems.
Throughout the operation of an SOFC, each layer is often cycled between room temperature and its full operating temperature. This thermal cycling causes the housing materials to contract and expand according to their coefficients of thermal expansion. This expansion and contraction introduces thermal stresses that may be transferred through the seals and other structural components directly to the ceramic cell. These thermal stresses effectively reduce the service life of an SOFC by compromising the seals or breaking the structurally brittle ceramic cells. Difficulties arise in maintaining a sealing relationship between individual parts while accommodating the thermal cycling.
Throughout the drawings, identical reference numbers designate similar, but not necessarily identical, elements.